Semiconductor device, manufacturing method thereof, and semiconductor manufacturing apparatus

ABSTRACT

An a-Si film is patterned into a linear shape (ribbon shape) or island shape on a glass substrate. The upper surface of the a-Si film or the lower surface of the glass substrate is irradiated and scanned with an energy beam output continuously along the time axis from a CW laser in a direction indicated by an arrow, thereby crystallizing the a-Si film. This implements a TFT in which the transistor characteristics of the TFT are made uniform at high level, and the mobility is high particularly in a peripheral circuit region to enable high-speed driving in applications to a system-on glass and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims priority of Japanese PatentApplication Nos. 2000-255646 and 2001-202730, filed on Aug. 25, 2000 andJul. 3, 2001, the contents being incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices, manufacturingmethods of the same, and semiconductor manufacturing apparatus, inparticular, for being suitably applied to a so-called system-on panel,in which a pixel region including thin film transistors and a peripheralcircuit region including thin film transistors are formed on anon-crystallized (amorphous) substrate such as a non-alkali glasssubstrate.

2. Description of the Related Art

A TFT (Thin Film Transistor) is formed on a very thin, fine activesemiconductor film. The TFT is examined to be mounted on a large-screenliquid crystal panel or the like in consideration of recent demands foran increase in area. In particular, applications to a system-on paneland the like are expected.

On the system-on panel, polycrystalline semiconductor TFTs (especiallypolysilicon TFTs (p-Si TFTs)) are formed on a non-crystallized substratesuch as a non-alkali glass substrate. In this case, as a popular method,an amorphous silicon (a-Si) film is formed as a semiconductor film, andthen irradiated with an ultraviolet short-pulse excimer laser to fuseand crystallize only the a-Si film without influencing the glasssubstrate, thereby obtaining a p-Si film functioning as an activesemiconductor film.

Excimer lasers which emit high-output linear beams coping with a largearea of the system-on panel have been developed. A p-Si film obtained byexcimer laser crystallization is readily influenced by not only theirradiation energy density but also the beam profile, the state of thefilm surface, or the like. It is difficult to form uniformly a p-Si filmlarge in crystal grain size in a large area. A sample crystallized by anexcimer laser was observed with an AFM to find that crystal grainsisotropically growing from nuclei produced at random exhibited a shapeclose to a regular polygonal shape, projections were observed at acrystal grain boundary at which crystal grains collided against eachother, and the crystal grain size was less than 1 μm, as shown in FIG.37.

In this manner, when a TFT is fabricated using a p-Si film obtained bycrystallization using an excimer laser, a channel region contains manycrystal grains. If the crystal grain size is large, and the number ofgrain boundaries present in the channel is small, the mobility is high.If the crystal grain size at a channel region portion is small, and thenumber of grain boundaries present in the channel is large, the mobilityis low. Thus, the transistor characteristics of the TFT readily varydependently on the grain size. In addition, the crystal grain boundarieshave many defects, and the presence of the grain boundaries in thechannel suppresses transistor characteristics. The mobility of the TFTattained by All this technique is about 150 cm²/Vs.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide semiconductordevices including TFTs in which the transistor characteristics of theTFTs are made uniform at a high level, and the mobility is highparticularly in a peripheral circuit region to enable high-speed drivingin applications to peripheral circuit-integrated TFT-LCDs, system-onpanels, system-on glasses, and the like.

It is another object of the present invention to provide semiconductordevices in which the insufficiency of the output of an energy beam,which outputs energy continuously in relation to time, is compensated sothat the throughput in crystallization of semiconductor films isimproved, thereby realizing highly efficient TFTs whose mobility is highparticularly in a peripheral circuit region to enable high-speeddriving.

It is still another object of the present invention to providemanufacturing methods of such semiconductor devices.

It is still another object of the present invention to provide apparatusfor manufacturing such semiconductor devices.

According to the first aspect of the present invention, there isprovided a method of manufacturing a semiconductor device in which apixel region having thin film transistors and a peripheral circuitregion are formed on a non-crystallized substrate, comprisingcrystallizing a semiconductor film formed in the peripheral circuitregion with an energy beam which outputs energy continuously along atime axis at least for the peripheral circuit region, thereby formingthe semiconductor film into active semiconductor films of the respectivethin film transistors.

In this case, the energy beam is preferably a CW laser beam, morepreferably, a solid state laser beam (DPSS (Diode Pumped Solid StateLaser) laser beam).

By crystallizing a semiconductor film with an energy beam which outputsenergy continuously along the time axis, the crystal grain size isincreased, e.g., the crystalline state of the semiconductor film isformed into a streamlined flow pattern having long crystal grains in theenergy beam scan direction. The crystal grain size in this case is 10 to100 times the size obtained by crystallization using a currentlyavailable excimer laser.

In the first aspect, each semiconductor film is preferably patternedinto a linear or island shape on the non-crystallized substrate.

The crystallization technique using a CW laser has conventionally beenstudied in the SOI field, but a glass substrate has been considered notto resist heat. When a glass substrate is irradiated with a laser whilean a-Si film is formed as a semiconductor film on the entire surface,the temperature of the glass substrate rises along with the temperaturerise of the a-Si film, and damage such as cracks is observed. In thepresent invention, the semiconductor film is processed into a linear orisland shape in advance to prevent the temperature rise of the glasssubstrate, generation of cracks, and diffusion of impurities into afilm. Even in forming the active semiconductor film of a TFT on anon-crystallized substrate such as a glass substrate, an energy beamwhich outputs energy continuously along the time axis from a CW laser orthe like can be used without any problem.

In the first aspect, an energy beam irradiation positioning markercorresponding to each patterned semiconductor film is formed on thenon-crystallized substrate.

This marker can suppress an irradiation position shift of the energybeam. Supply of a stable continuous beam enables so-called lateralgrowth, and a semiconductor film having large-size crystal grains can bereliably formed.

In the first aspect, it is preferable that slits be formed in eachsemiconductor film patterned on the non-crystallized substrate, orthin-line insulating films be formed on each semiconductor film, and thesemiconductor film be irradiated with the energy beam in an almostlongitudinal direction of the slits.

In this case, the slits or insulating films (to be simply referred to asslits hereinafter for convenience) block crystal grains and grainboundaries which grow inward from the periphery in crystallization byirradiation of an energy beam. Only crystal grains which grow parallelto the slits are formed between the slits. If the region between theslits is satisfactorily narrow, single crystals are formed in thisregion. In this manner, the channel region can be selectively changedinto a monocrystalline state by forming the slits so as to set a regionwhere large-size crystal grains are to be formed, e.g., the regionbetween the slits as the channel region of a semiconductor element,e.g., thin film transistor.

In the first aspect, it is preferable that an irradiation condition ofthe energy beam which outputs energy continuously along the time axis bechanged between the pixel region and the peripheral circuit region, thata semiconductor film formed in the pixel region be crystallized with anenergy beam which outputs energy pulses, and the semiconductor filmformed in the peripheral circuit region be crystallized with the energybeam which outputs energy continuously along the time axis (morespecifically, the semiconductor film formed in the pixel region becrystallized, and then the semiconductor film formed in the peripheralcircuit region be crystallized), or that the semiconductor film formedin the peripheral circuit region be crystallized with the energy beamwhich outputs energy continuously along the time axis, the crystallizedsemiconductor film be set as an active semiconductor film, and asemiconductor film formed in the pixel region be set as an activesemiconductor film without any change.

Although positional controllability is important for either of the pixeland peripheral regions, any thin film transistor formed in theperipheral circuit region requires higher performance than that in thepixel region, and must be optimized in fabrication. For this purpose, anenergy beam which continuously outputs energy and can reliably form anactive semiconductor film having large-size crystal grains and make theoperation characteristics of respective thin film transistors uniform athigh level is applied particularly to the peripheral circuit region. Inthe pixel region where the required performance is low, the energy beamirradiation time is shortened, or a pulse-like energy beam is applied.In this way, the crystallization process is changed between theperipheral circuit region and the pixel region. Accordingly, a desiredsystem-on panel which very efficiently satisfies the requiredperformance of respective locations can be implemented.

According to the second aspect of the present invention, there isprovided a semiconductor device in which a pixel region having thin filmtransistors and a peripheral circuit region are formed on anon-crystallized substrate, wherein active semiconductor films of therespective thin film transistors constituting at least the peripheralcircuit region are formed into a crystalline state of a streamlined flowpattern having large crystal grains.

In this case, the active semiconductor film can be changed into alarge-crystal-grain state, and preferably a monocrystalline state alongthe streamlined shape of the flow pattern. For example, the channelregion of a thin film transistor can be changed into a monocrystallinestate. A high-speed-driving thin film transistor excellent in transistorcharacteristics can be implemented.

Besides, the semiconductor film is preferably formed over thenon-crystallized substrate with a buffer layer being interposed betweenthem. The buffer layer includes a thin film containing Si and N, or Si,O, and N. The density of hydrogen in the semiconductor film ispreferably 1×10²⁰/cm³ or less, more preferably, the density of hydrogenin the buffer layer is 1×10²²/cm³ or less.

By this construction, the transistor characteristics of the TFTs can beuniformized at a high level using crystallization with an energy beamthat outputs energy continuously in relation to time. Further, the TFTscan stably be formed without generation of pinholes or peeling-off. Veryhighly reliable TFTs can be realized thereby.

According to the third aspect of the present invention, there isprovided a semiconductor manufacturing apparatus for emitting an energybeam for crystallizing a semiconductor film formed on a non-crystallizedsubstrate, wherein the semiconductor manufacturing apparatus can outputthe energy beam continuously along a time axis, and has a function ofscanning the energy beam to an object to be irradiated, and outputinstability of the energy beam has a value smaller than ±1%.

In this case, the output instability of the energy beam is set to avalue smaller than ±1%, and more preferably noise representing theinstability of the energy beam with respect to the time is set to 0.1rms % or less. A stable continuous beam can therefore be supplied. Thecontinuous beam can be scanned to form uniformly the activesemiconductor films of many thin film transistors in a large-sizecrystalline state (flow pattern).

According to the fourth aspect of the present invention, like the thirdaspect, there is provided a semiconductor manufacturing apparatus. Theapparatus comprising disposing means for disposing a non-crystallizedsubstrate on a surface of which a semiconductor film is formed, so thatthe non-crystallized substrate can freely be moved in a plane parallelwith a surface of the semiconductor film, laser oscillation means thatcan output an energy beam continuous in relation to time, and beamsplitting means for optically splitting the energy beam emitted from thelaser oscillation means, into sub-beams. Each of the sub-beams isapplied to relatively scan the corresponding portion of thesemiconductor film to crystallize.

In this case, with the split sub-beams, the predetermined portions ofthe semiconductor film corresponding to the respective sub-beams can becrystallized at once. Thus, each of the active semiconductor films ofmany thin film transistors can be formed uniformly in a large-sizecrystalline state (flow pattern). Besides, even when using laseroscillation means whose output is lower than those of excimer lasers,such as a CW laser, a very high throughput not inferior to those in caseof using excimer lasers can be obtained. Crystallization for thin filmtransistors can efficiently be achieved thereby.

In the fourth aspect, each sub-beam is preferably controlled so thatonly the portion where a thin film transistor is to be formed isirradiated with the beam at the optimum energy intensity forcrystallization and the beam rapidly pass the portion where no thin filmtransistor is to be formed. A higher throughput can be obtained thereby,and very highly efficient crystallization for thin film transistors canbe realized.

According to the fifth aspect of the present invention, like the thirdaspect, there is provided a semiconductor manufacturing apparatus. Theapparatus comprising disposing means for disposing a non-crystallizedsubstrate on a surface of which a semiconductor film is formed, so thatthe non-crystallized substrate can freely be moved in a plane parallelwith a surface of the semiconductor film, laser oscillation means thatcan output an energy beam continuous in relation to time, andintermittent (pulse) emission means having a transmission area and aninterruption area for the energy beam to intermittently transmit theenergy beam. With moving the energy beam to relatively scan thenon-crystallized substrate, the energy beam is intermittently applied tothe semiconductor film to selectively crystallize only the portion wherea thin film transistor is to be formed.

In this case, by controlling the transmission of the energy beam mainlywith the intermittent emission means, only desired portions of thesemiconductor film can be selectively crystallized. That is, onlydesired portions of the semiconductor film in the non-patterned statecan be selectively crystallized. Therefore, it is needless to set up inadvance the portions to be irradiated with the beam, i.e., the portions(ribbon-like or island-like) where thin film transistors are to beformed. As a result, the number of manufacturing steps can be reducedand the throughput can be improved.

In the fifth aspect, it is preferable that the energy beam isintermittently applied to certain portions other than where thin filmtransistors are to be formed, so as to form positioning markers for thethin film transistors each crystallized into a predetermined shape. Byforming the positioning markers simultaneously with the crystallizationof where thin film transistors are to be formed, the number ofmanufacturing steps can be reduced and efficient and accurate formationof the thin film transistors becomes possible.

The present invention includes semiconductor devices and manufacturingmethods of the semiconductor devices, corresponding to the above fourthand fifth aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic plan views showing the crystallizing stateof a semiconductor film in the first embodiment of the presentinvention;

FIGS. 2A and 2B are a plan view and a photomicrographic view,respectively, showing the state of a semiconductor film patterned into aribbon shape;

FIGS. 3A and 3B are photomicrographic views showing a state in which aTFT island is formed;

FIG. 4 is a SEM view showing the state of a semiconductor filmcrystallized by a CW laser (flow pattern);

FIG. 5 is a SEM view showing the state of a semiconductor filmcrystallized into an excimer pattern by a CW laser;

FIG. 6 is a graph showing SIMS analysis around a semiconductor film;

FIG. 7 is a photographic view showing sectional TEM around asemiconductor film;

FIGS. 8A to 8C are schematic sectional views, respectively, showing thesteps in manufacturing a TFT according to the first embodiment;

FIGS. 9A to 9C are schematic sectional views, respectively, showing thesteps subsequent to FIG. 8C in manufacturing a TFT according to thefirst embodiment;

FIGS. 10A and 10B are schematic sectional views, respectively, showingthe steps subsequent to FIG. 9C in manufacturing a TFT according to thefirst embodiment;

FIGS. 11A to 11C are schematic sectional views, respectively, showingthe steps subsequent to FIG. 10B in manufacturing a TFT according to thefirst embodiment;

FIG. 12 is a graph showing the relationship between the crystal patternand mobility of a semiconductor film;

FIG. 13 is a photomicrographic view showing the relationship between thecrystal pattern and mobility of the semiconductor film;

FIG. 14 is a schematic plan view showing ribbon-shaped semiconductorfilms and position markers in the first modification of the firstembodiment;

FIGS. 15A to 15D are schematic plan views showing the states of asemiconductor film in the second modification of the first embodiment;

FIGS. 16A to 16D are schematic views showing the states of asemiconductor film in the third modification of the first embodiment;

FIG. 17 is a schematic plan view showing the states of a semiconductorfilm in the fourth modification of the first embodiment;

FIGS. 18A to 18C are schematic views showing the states of asemiconductor film in the fifth modification of the first embodiment;

FIG. 19 is a schematic view showing a DPSS laser device in the secondembodiment of the present invention;

FIG. 20 is a schematic view showing a DPSS laser device in the firstmodification of the second embodiment;

FIG. 21 is a schematic view showing a DPSS laser device in the secondmodification of the second embodiment;

FIG. 22 is a schematic view showing part of a DPSS laser deviceaccording to the third embodiment of the present invention;

FIG. 23 is a schematic view showing a state that 28 sub-beams in totalare generated using four DPSS lasers;

FIGS. 24A and 24B are a schematic view showing another irradiationmethod using the DPSS lasers;

FIG. 25 is a schematic view showing a state of selectively crystallizinga semiconductor film only at regions where TFTs are to be formed;

FIGS. 26A and 26B are schematic views showing irradiation systems ofDPSS lasers used in modifications of the third embodiment;

FIG. 27 is a schematic view showing the principal part of a DPSS laserdevice according to the fourth embodiment of the present invention;

FIG. 28 is a schematic view showing an example of arrangement of TFTs ina pixel region;

FIG. 29 is a schematic view showing the principal part of a DPSS laserdevice according to modification 2 of the fourth embodiment;

FIG. 30 is a schematic view showing the principal part of a DPSS laserdevice according to modification 3 of the fourth embodiment;

FIG. 31 is a schematic view showing the principal part of a DPSS laserdevice according to modification 4 of the fourth embodiment;

FIG. 32 is a graph showing a result obtained by examining thedistribution of hydrogen in a buffer layer made of SiN or SiON, and a Silayer;

FIG. 33 is a photomicrographic view showing a state of an a-Si filmpeeling off;

FIG. 34 is a schematic sectional view showing a state that an a-Si filmis formed over a glass substrate with a buffer layer being interposedbetween them;

FIG. 35 is a graph showing an result of an SIMS analysis of a glasssubstrate/SiN/SiO₂/a-Si structure after a thermal treatment at 500° C.for two hours;

FIG. 36 is a photomicrographic view showing a semiconductor film aftercrystallization; and

FIG. 37 is an AFM view showing a state of a silicon film crystallizedusing a conventional excimer laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments to which the present invention is applied will bedescribed below in detail with reference to drawings.

(First Embodiment)

Crystallization by Energy Beam Output Continuously along Time Axis

The principal part of the first embodiment of the present invention,i.e., crystallization of a semiconductor film using an energy beam whichoutputs energy continuously along the time axis, e.g., a solid statelaser (DPSS (Diode Pumped Solid State Laser) laser) of semiconductorexcitation (LD excitation) will be disclosed.

An energy beam continuous along the time axis can irradiate and scan,e.g., an amorphous silicon (a-Si film) to form large-size polysiliconcrystals. The crystal grain size at this time is about several μm, andvery large crystals can be formed. This crystal grain size is 10 to 100times the size obtained by using a currently available excimer laser.Hence, such crystals are very advantageous to TFTs at a peripheralcircuit portion required to operate at high speed.

As shown in FIGS. 1A to 2B, an a-Si film 2 is patterned into a linear(ribbon) shape (FIG. 1A) or an island shape (FIG. 1B) on a glasssubstrate 1 having an SiO₂ buffer formed thereon. The upper surface ofthe a-Si film 2 or the lower surface of the glass substrate 1 isirradiated and scanned with an energy beam 3 output continuously alongthe time axis from a CW laser in a direction indicated by an arrow.After this, as shown in FIGS. 3A and 3B, each ribbon-shapedsemiconductor film 2 (FIG. 3A) or each island-shaped semiconductor film2 (FIG. 3B) is patterned and etched to form a TFT island region 6 havingsource and drain regions 5 via a channel region 4 in the semiconductorfilm 2.

Microcrystals are formed around the island region 6 because the coolingrate is high due to thermal diffusion to the peripheral region. However,the cooling rate in the island region 6 can be set sufficiently low byproperly selecting the irradiation conditions (energy and scan speed) ofthe CW laser 3, and crystal grains several μm in width and several tenμm in length are formed. Thus, the crystal grain size of the channelportion can be increased.

The crystallization technique using an energy beam continuous along thetime axis has conventionally been studied in the SOI (Silicon OnInsulator) field, but a glass substrate has been considered not toresist heat. When a glass substrate is irradiated with a laser while ana-Si film is formed on the entire surface, the temperature of the glasssubstrate rises along with the temperature rise of the a-Si film, anddamage such as cracks is observed. To solve this, the a-Si film isprocessed into a ribbon or island shape in advance to prevent thetemperature rise of the glass substrate, generation of cracks, anddiffusion of impurities into the film.

To form many TFTs in a large area, the energy beam must be stable. Asolid state laser of semiconductor LD excitation has a stability of 0.1rms % noise or less and an energy stability of <±1%/h, and is superiorto other energy beams.

An example of crystallization using a diode pumped solid state laser(DPSS laser) with semiconductor excitation (LD excitation) will bedescribed below.

The wavelength of the solid state laser is 532 nm (the second harmonicof Nd:YVO₄, the second harmonic of Nd:YAG, or the like). The energy beamoutput stability is <0.1 rms % noise, and the output time stability is<±1%/h. Note that the wavelength is not limited to this, and anywavelength capable of crystallizing a semiconductor film may be used.The output is 10 W, and NA35 glass is used as a non-crystallizedsubstrate. The material of the non-crystallized substrate is not limitedto this, and may be another non-alkali glass, silica glass,single-crystal silicon with amorphous insulating layer, ceramics,plastic, or the like.

An SiO₂ buffer layer is formed at a film thickness of about 400 nmbetween a glass substrate and a semiconductor film. Note that the bufferlayer is not limited to this, and may be a layered structure of an SiO₂film and SiN film. The go semiconductor film is a silicon thin filmformed by plasma CVD. Heat treatment for dehydrogenation is done at 450°for 2 h before energy irradiation. Dehydrogenation is not limited toheat treatment, and may be achieved by emitting an energy beam manytimes while gradually increasing the energy level from a low energyside. In this embodiment, the semiconductor film is irradiated from thelower surface via the glass, but is not limited to this. Thesemiconductor film may be irradiated from the semiconductor film side.

The energy beam is shaped into an elongated linear beam (rectangularbeam) 400 μm×40 μm in size. The size and shape of the energy beam arenot limited to them, and the energy beam may be adjusted to an optimalsize necessary for crystallization. As the shape of the beam, arectangular (or elliptic) beam, a linear (or elliptic) beam, or thelike, can be suitably used. Although it is preferable that such a longlinear (or elliptic) beam, rectangular (or elliptic) beam, or linear (orelliptic) beam has uniform distribution of energy intensity in the beam,it is not always required. Such a beam may have an energy profile inwhich the maximum intensity is at the center of the beam.

In this embodiment, as shown in FIGS. 2A and 2B, the a-Si film 2 ispatterned into a ribbon shape in the silicon region where a TFT isformed. Adjacent ribbon-shaped a-Si films 2 are separated by apredetermined distance, and a region where no a-Si film 2 exists ispresent. This layout of the a-Si films 2 can greatly reduce thermaldamage to the NA35 glass substrate 1. Note that the a-Si film is notlimited to the ribbon shape, but may be patterned into an island shape.

FIG. 4 shows the results of crystallizing an a-Si film at an energy beamscan speed of 20 cm/s.

Crystals 5 μm or more in crystal grain size are found to be formed. Thiscrystal grain size corresponds to a size 10 to 100 times the grain sizeof crystallization by an excimer laser. Crystal grains as if they flowin a scan direction are observed, and this crystal pattern is defined asa “flow pattern” in this embodiment. The name is not limited to this,and is defined for descriptive convenience in this embodiment. As acrystal grain size different from that of the flow pattern, a pattern asshown in FIG. 5 similar to a crystallization pattern by an excimer laserin FIG. 37 is sometimes formed. This crystal grain pattern is defined asan “excimer pattern” in this embodiment. The excimer pattern is formeddue to an improper energy density or scan speed (or both of them).

The results of observing the influence of a large amount of impuritiespresent in glass on a crystallized film will be described.

In this embodiment, the SiO₂ film about 400 nm in film thickness formedby PECVD is interposed between the NA35 glass substrate 1 and the a-Sifilm 2 serving as a semiconductor film. The buffer layer is not limitedto this, and may be 200 nm or more in film thickness for single SiO₂ ormay use a layered structure of SiO₂ film and SiN film.

FIG. 6 shows the results of SIMS analysis.

It is confirmed that impurities (aluminum, boron, sodium, and barium) inglass do not exist in a crystallized semiconductor thin film. Aluminumis observed in data, but is a ghost. Aluminum does not actually exist inthe film. The density of sodium is lower than detection limit.

FIG. 7 shows the results of inspecting thermal damage to NA35 glass(results of observing sectional TEM).

As shown in FIG. 7, the interface between the glass and the buffer layeris definite, and no damage to the glass can be confirmed.

In this embodiment, crystallization is achieved using one DPSS laserhaving an output of 10 W and a wavelength of 532 nm. When the layout ofa semiconductor thin film pattern is already known, as shown in FIGS. 2Aand 2B, it is possible to form plural beams and simultaneously emit themwhile matching them with semiconductor thin film regions. At this time,plural energy beam generators may be used, or an energy beam from onegenerator may be split into beams.

Fabrication of TFT

An example of fabrication of an n-channel thin film transistor using theabove-described energy beam output continuously along the time axis willbe explained. FIGS. 8A to 11C are schematic sectional views,respectively, showing the steps in manufacturing the thin filmtransistor.

As a non-crystallized substrate, an NA35 glass substrate 21 is usedsimilarly to the above example. As shown in FIG. 8A, an SiO₂ bufferlayer 22 about 400 nm in film thickness, and a patterning Si thin filmhaving a non-crystallized silicon thin film are formed on the glasssubstrate 21. Heat treatment is done at 450° for 2 h for the sake ofdehydrogenation. Dehydrogenation is not limited to heat treatment, andmay be achieved by emitting an energy beam many times while graduallyincreasing the energy level from a low energy side.

Then, the a-Si film 2 is crystallized using the above-mentioned energybeam output continuously along the time axis, thereby forming an activesemiconductor film 11.

More specifically, as shown in, e.g., FIG. 2A, a semiconductor film, inthis case the a-Si film 2 is formed in a ribbon shape. A DPSS laser isused with a wavelength of 532 nm, an energy beam stability of <0.1 rms %noise and an output stability of ±1%/h. The a-Si film 2 is crystallizedby irradiating and scanning it by a linear beam 400 μm×40 μm in energybeam size at a scan speed of 20 cm/s.

Subsequently, as shown in, e.g., FIG. 3A, a TFT island region 6 isformed in the crystallized ribbon-shaped semiconductor film. At thistime, the semiconductor film is so processed as to position a TFTchannel region 4 on the central axis of the ribbon-shaped semiconductorfilm. That is, a current flowing in a completed TFT coincides with thelaser beam scan direction. In this case, as shown in the lower part ofFIG. 2A, a plurality (three in the illustrated example) of TFTs may beformed in each ribbon width.

As shown in FIG. 8B, a silicon oxide film 23 serving as a gate oxidefilm is formed to a film thickness of about 200 nm on the activesemiconductor film 11 by PECVD. The silicon oxide film 23 may be formedby another method such as LPCVD or sputtering.

As shown in FIG. 8C, an aluminum film (or aluminum alloy film) 24 issputtered to a film thickness of about 300 nm.

As shown in FIG. 9A, the aluminum film 24 is patterned into an electrodeshape by photolithography and dry etching, thereby forming a gateelectrode 24.

As shown in FIG. 9B, the silicon oxide film 23 is patterned using thepatterned gate electrode 24 as a mask, thereby forming a gate oxide film23 conforming to the gate electrode shape.

As shown in FIG. 9C, ions are implanted in the two sides of the gateelectrode 24 of the active semiconductor film 11 using the gateelectrode 24 as a mask. More specifically, n-type impurities, in thiscase phosphorus (P) are ion-implanted at an acceleration energy of 20keV and a dose of 4×10¹⁵/cm², thereby forming source and drain regions.

As shown in FIG. 10A, the resultant structure is irradiated with anexcimer laser in order to activate phosphorous in the source and drainregions. Then, as shown in FIG. 10B, SiN is deposited to a filmthickness of about 300 nm so as to cover the entire surface, therebyforming an interlevel insulating film 25.

As shown in FIG. 11A, contact holes 26 for exposing the gate electrode24 and the source and drain regions of the active semiconductor film 11are formed in the interlevel insulating film 25.

As shown in FIG. 11B, metal films 27 of aluminum or the like are formedto bury the contact holes 26. As shown in FIG. 11C, the metal films 27are patterned to form wiring lines 27 which connect the gate electrode24 and the source and drain regions of the active semiconductor film 11via the contact holes 26. After that, formation of a protection filmwhich covers the entire surface, and the like are performed to completethe n-type TFT.

The relationship between TFT characteristics and the crystal quality waschecked using the n-channel TFT fabricated by the above steps. FIG. 12shows experimental results.

When the crystal pattern of the channel region is a flow pattern, themobility is higher than that in an excimer laser pattern. The highestmobility is 470 cm²/Vs. The mobility is strongly related to the flowpattern shape. As shown in FIG. 13, the mobility is confirmed to behigher in a strong flow pattern shape than in a weak flow pattern shape.

As described above, this embodiment can implement TFTs in which thetransistor characteristics of the TFTs are made uniform at high level,and the mobility is high particularly in a peripheral circuit region toenable high-speed driving. This can implement a high-performance,peripheral circuit-integrated TFT-LCD, system-on panel, or system-onglass having many TFTs.

Modifications

Modifications of the first embodiment will be described.

(First Modification)

FIG. 14 is a schematic plan view showing a state on a glass substrate inthe first modification.

In this modification, the ribbon-shaped a-Si films 2 are formed assemiconductor films on the glass substrate 1, and a position marker 31is set at an end portion of the glass substrate 1 corresponding to eacha-Si film 2. Although the a-Si films are ribbon-shaped in theillustrated example, they may alternatively be island-shaped.

In irradiating and scanning the a-Si film 2 with an energy beam from theCW laser 3, the irradiation position can be automatically searched withreference to the position marker 31. After the irradiation position isdetermined, the energy beam is scanned to perform crystallization.

This modification can suppress a shift in the irradiation position ofthe energy beam. Supply of a stable continuous beam enables so-calledlateral growth, and an active semiconductor film having large-sizecrystal grains can be reliably formed.

(Second Modification)

FIGS. 15A to 15D are schematic plan views for explaining the secondmodification.

As shown in FIG. 15A, the a-Si film is processed into an island shapehaving two almost parallel slits 32 and an island region is formed.

The upper surface of the a-Si film is irradiated with a CW laser, e.g.,Nd:YVO₄ laser (2ω, wavelength of 532 nm) in the direction of the slit 32(indicated by an arrow) at an energy of 6 W, a beam size of 400 μm×40μm, and a scan speed of 20 cm/s. Even irradiation from the upper surfacecan normally crystallize the a-Si film. If the a-Si film is irradiatedfrom the lower surface, the sample holder is also heated to obtain aheat-insulating effect on the film surface side and easily obtainhigher-quality crystals. The a-Si film is fused and crystallized. Thecooling rate is high around the island region 6 owing to thermaldiffusion to the peripheral region, so that microcrystals are formed. Inthe channel region 4, the cooling rate can be set sufficiently low byproperly selecting the irradiation conditions (energy and scan speed) ofthe CW laser, and crystal grains several μm in width and several ten μmin length are formed.

At this time, as shown in FIG. 15B, the slits 32 block crystal grainsand grain boundaries which grow inward from the periphery and are tocross the channel region. Only crystal grains which grow parallel to theslits 32 are formed between the slits 32. If the interval between theslits 32 is satisfactorily small, this region is formed from singlecrystals. The slit width of each slit 32 is preferably formed as smallas possible so as not to change the region between the slits 32 intomicrocrystals, while the slit 32 holding the grain boundary blockingeffect. The interval between the slits 32 is set with a margin inaccordance with the channel width of the device.

As shown in FIG. 15C, the resultant structure is patterned by dryetching so as to set the single-crystal portion between the slits 32 asthe channel region 4, thereby completing the TFT.

As shown in FIG. 15D, a gate insulating film and gate electrode areformed by a known method. After impurities are implanted and activated,a source and drain are formed to fabricate the TFT.

By crystallization using this method, single crystals can be selectivelyobtained at a necessary portion of the channel region of the TFT. In aTFT formed using an active semiconductor film prepared in this way, onlyone crystal grain exists in the channel region. Thus, thecharacteristics are improved, and variations caused by crystallinity andthe crystal grain boundary are greatly reduced. Further, variousprocesses on a glass substrate can be performed, and a high-performancedisplay with high value added at low cost can be provided.

(Third Modification)

FIGS. 16A to 16D are schematic plan views for explaining the thirdmodification, and schematic sectional views taken along the line I-I′.

After the SiO₂ underlayer and a-Si film 2 are successively formed on theglass substrate 1, the a-Si film 2 is patterned into an island shape, asshown in FIG. 16A.

As shown in FIG. 16B, an SiO₂ film is formed to a film thickness ofabout 50 nm on the a-Si film 2 by CVD or the like, and processed intotwo parallel thin-line patterns 33.

As shown in FIG. 16C, the a-Si film 2 is irradiated and scanned with aCW laser from the upper surface. The irradiation conditions are the sameas in the first embodiment. At this time, the a-Si film 2 is fused andcrystallized again by laser heating. However, since the thin-linepatterns 33 exist on the a-Si film 2, fused Si easily gathers by thesurface tension, and Si thin lines 33 a independent of the periphery areformed at the lower portions of the thin-line patterns 33. The Si thinlines block crystal grains and crystal grain boundaries which are tocross the channel. As a result, only crystal grains which grow parallelto the thin lines are formed between the two thin-line patterns 33.

The SiO₂ films of the thin-line patterns 33 are removed with an aqueousHF solution or the like. As shown in FIG. 16D, the resultant structureis processed by dry etching so as to set the single-crystal portionbetween the thin-line patterns 33 as the channel region 4. Thereafter, agate insulating film and gate electrode are formed by a known method,thereby fabricating the TFT.

By crystallization using this method, single crystals can be selectivelyobtained at a necessary portion of the channel region of the TFT. In aTFT formed using an active semiconductor film prepared in this manner,only one crystal grain exists in the channel region. The characteristicsare improved, and variations caused by crystallinity and the crystalgrain boundary are greatly reduced. Moreover, various processes on aglass substrate can be performed, and a high-performance display withhigh value added at low cost can be provided.

(Fourth Modification)

The fourth modification is almost the same in the manufacturing processas the second modification except for the slit shape. FIG. 17 shows aslit shape in the fourth modification. Unlike the slit shape in FIGS.15A to 15C, two slits 32 are not completely parallel, but are slightlywidened in the laser scan direction. This shape can more efficientlyblock crystal grain boundaries diagonally inward from the periphery. Inaddition, crystal grains extending from the lower side in the drawingcan be easily selected by the necking effect. The subsequent process isthe same as in the second modification.

(Fifth Modification)

FIGS. 18A to 18C are schematic views for explaining the fifthmodification. The upper portion of FIG. 18A is a plan view of apatterning portion, and the lower portion is a sectional view takenalong the line I-I′. FIGS. 18B and 18C show manufacturing stepssubsequent to FIG. 18A.

In an a-Si film 2, a thin film region 34 is surrounded by a thick-filmregion 35. Scanning and irradiation with a CW laser are executed in thelongitudinal direction of the thin film region 34 (see FIG. 18A). Atthis time, the thick-film region 35 has a large heat capacity due to itsthickness, and decreases in cooling rate during solidification. Hence,the thick-film region 35 has a hot bath effect on the thin film region34. In the thin film region 34, the crystal grain boundary spreadstoward the peripheral thick-film region 35 (see FIG. 18B). This meansthat the defect (crystal grain boundary) density decreases in the thinfilm region 34. That is, high-quality crystals can be implemented.

By using the thin film region 34 as the channel region of the TFT, ahigh-performance TFT can be implemented (see FIG. 18C).

(Second Embodiment)

The second embodiment of the present invention will be described below.

The construction of the DPSS laser device used in the first embodimentwill be explained.

FIG. 19 is a view showing the outer appearance of the whole constructionof a DPSS laser device according to the second embodiment.

This DPSS laser device comprises a DPSS laser 41 of solid semiconductorLD excitation, an optical system 42 for irradiating a predeterminedposition with a laser beam emitted by the DPSS laser 41, and an X-Ystage 43 freely drivable in horizontal and vertical directions to whicha glass substrate to be irradiated is fixed.

In the second embodiment, the glass substrate is made of NA35 glass(non-alkali glass), and the laser wavelength is 532 nm. Outputvariations in energy beam are noise of 0.1 rms % or less, the outputinstability is <±1%/h, and the output of the energy beam is 10 W. Notethat the wavelength is not limited to this value, and any wavelengthcapable of crystallizing a silicon film may be used. Also, the beamoutput is not limited to this value, and a device having an appropriateoutput may be used.

The energy beam is shaped into a linear beam (rectangular beam) 400μm×40 μm in size. The size and shape of the energy beam are not limitedto them, and the energy beam may be adjusted to an optimal sizenecessary for crystallization. Energy variations in the longitudinaldirection are within 40% using the center as the maximum intensity.

The glass substrate is set on the X-Y stage 43 to be perpendicular tothe optical axis.

In the second embodiment, as well as the first embodiment, asemiconductor film (a-Si film) on which a TFT is to be fabricated isformed into a ribbon or island shape, as shown in FIG. 1A or 1B.Adjacent a-Si films are separated, and a region where no a-Si filmexists is present. This reduces thermal damage to the glass substrateemployed in this embodiment.

The energy beam scan speed is 20 cm/s. This embodiment adopts themotor-driven X-Y stage 43. Note that the driving mechanism of the X-Ystage 43 is not limited to this, and another stage can be used as far asit can be driven at a speed of 15 cm/s or more. Scanning with the energybeam can be performed by moving the energy beam relatively to the X-Ystage 43. That is, either the energy beam or the X-Y stage can be moved.

When polysilicon is to be formed on the glass substrate, positionalcontrol during scan is important because the current substrate size is400 mm×500 mm. The X-Y stage 43 of this embodiment exhibits a positionalvariation of 10 μm or less every one-meter movement.

The DPSS laser device of this embodiment can supply a stable continuousbeam by setting the energy beam output instability to a value smallerthan ±1%, and preferably setting noise representing the energy beaminstability with respect to the time to 0.1 rms % or less. By scanningthis continuous beam, the active semiconductor films of many TFTs can beuniformly formed in a crystalline state (flow pattern) having a largegrain size.

Modifications

Modifications of the second embodiment will be described below.

(First Modification)

FIG. 20 shows the whole construction of a DPSS laser device according tothe first modification.

This modification uses the two DPSS lasers 41 having an output stabilityof 0.1 rms % noise or less, an output instability of <±1%/h, and anoutput of 10 W. Laser beams emitted by the two DPSS lasers 41 aremultiplexed into one midway along the optical path, thereby improvingthe output.

The beam size is set to 800 μm×40 μm so as to irradiate a larger areathan in the second embodiment. Similar to the first embodiment, thismodification has a function of reading and irradiating a positionmarker.

The X-Y stage 43 is horizontally placed, and a glass substrate ishorizontally set. A magnetic levitation type moving mechanism isdisposed in the irradiation/scan direction, whereas a normal motordriving scheme is used in the X-axis direction. The energy beam isvertically emitted.

In addition to the effects of the second embodiment, the DPSS laserdevice of the first modification can supply a stabler continuous beam byproviding DPSS lasers 41 (two in this case). By scanning this continuousbeam, the active semiconductor films of many TFTs can be uniformlyformed in a crystalline state (flow pattern) having a large grain size.

(Second Modification)

FIG. 21 shows the whole construction of a DPSS laser device according tothe second modification.

This modification adopts the two DPSS lasers 41 having the same outputstability, output, and the like as in the first modification, andirradiates different locations. Each energy beam has a function ofreading an irradiation position from a position marker.

In addition to the effects of the second embodiment, the DPSS laserdevice of the second modification can rapidly supply a stablercontinuous beam by providing DPSS lasers 41 (two in this case). Byscanning this continuous beam, the active semiconductor films of manyTFTs can be uniformly formed in a crystalline state (flow pattern)having a large grain size.

(Third Embodiment)

Next, the third embodiment of the present invention will be described.

Here, like the second embodiment, the construction of a DPSS laserdevice and then a crystallization method of a semiconductor film usingthe DPSS laser device will be described. The DPSS laser device of thisembodiment differs from that of the second embodiment on the point thatthe energy beam is split as described below.

In this embodiment described is an example of crystallization in thepixel region using a semiconductor exciting (LD exciting) solid statelaser (DPSS laser) Nd:YVO₄. In this embodiment, a crystallizationtechnique for the pixel region will be described, but the presentinvention is not limited to this. The present invention is applicablealso to crystallization for any peripheral circuit. The laser is notlimited to Nd:YVO₄. A similar DPSS laser light (e.g., Nd:YAG or thelike) can be used also. The wavelength is 532 nm. The wavelength is notlimited to this. Any wavelength can be used if it can melt silicon. Thestability of the energy output is <0.1 rms % of noise, the stability oftime of the output is <±1%/h, and the output is 10 W.

As the non-crystallized substrate used is an NA35 glass substrate. Thenon-crystallized substrate is not limited to this. Another non-alkaliglass, quartz glass, monocrystalline substrate with amorphous insulatinglayer, ceramic, or plastic can be used.

An about 400 nm-thick buffer layer made of SiO₂ is formed between theglass substrate and a semiconductor film. The buffer layer is notlimited to this. It can be a multilayer film of SiO₂ and SiN. Thesemiconductor film is an about 150 nm-thick silicon film formed througha plasma CVD process. Before irradiation with an energy beam, a thermaltreatment for dehydrogenation has been performed at 500° C. for twohours. Dehydrogenation is not limited to such a thermal treatment. Itcan be implemented by many times of irradiation with an energy beam asthe energy of the beam is gradually increased from the lower energyside. In this embodiment, the irradiation is done from the semiconductorfilm side, but it may be done from the back surface side through theglass.

Construction of DPSS Laser Device

FIG. 22 is a schematic view showing part of a DPSS laser deviceaccording to this third embodiment.

The DPSS laser device includes a solid state semiconductor-exciting DPSSlaser 41 (not shown) like the second embodiment, a diffraction grating51 as beam splitting means for optically splitting an energy beamemitted from the DPSS laser 41, into a number of sub-beams, in theillustrated example, seven sub-beams, a collimator lens 52, a condenserlens 53 for condensing the split sub-beams, and an X-Y stage 43 (notshown), like the second embodiment, on which the glass substrate to beirradiated is mounted and which can freely be driven horizontally andvertically.

Although the diffraction grating 51 is used as the beam splitting meansin this embodiment, the beam splitting means is not limited to this. Forexample, also usable is a polygon mirror, a movable mirror, an AO device(Acousto-Optic Device) using an acoustooptic effect, or an EO device(Electro-Optic Device) using an electrooptic effect.

Each sub-beam has a size of 80 μm×20 μm, which is sufficient for forminga thin film transistor in the pixel region. Each sub-beam has anelliptic beam shape in which the maximum intensity is at the centroid.The beam shape is not limited to such an elliptic beam shape. A longlinear beam (or a rectangular beam) can be used also. The size of theenergy beam is also not limited to the above. Any beam size can be usedif it can form a pixel TFT.

In this embodiment, each silicon region where a pixel TFT is to beformed is ribbon-shaped as shown in FIG. 23. Each semiconductor filmribbon 54 is separated from the neighboring semiconductor film ribbon 54on either side, and there is interposed a region including nosemiconductor film portion. This is for reducing thermal damage on theNA35 glass substrate used in this embodiment.

The pattern of the semiconductor film is not limited to such a ribbonshape. An island pattern can be used also. Since such a pixel TFTrequires not so high performance, the beam energy density forcrystallization can be reduced in comparison with the peripheral region.For this reason, even if the amorphous silicon were formed on the entiresurface, crystallization can be performed without damaging the glasssubstrate.

FIG. 23 is a schematic view showing a state that 28 sub-beams in totalare generated using four DPSS lasers.

In the crystallization technique for pixel TFTs in this embodiment, thescanning speed with the energy beam is 100 cm/sec. The scanning speed isnot limited to this value. Any scanning speed can be used if it canimplement the required performance of the pixel TFTs.

In order to irradiate the entire surface of the pixel region, the 28sub-beams in the lump are moved in parallel to crystallize every 28lines. The entire surface of the pixel region is thus crystallized withan improved throughput. In this embodiment, this operation is performedby rapidly moving the X-Y stage 43. But, the present invention is notlimited to this manner. The 28 sub-beams can be moved in the lump whilethe X-Y stage 43 is fixed. The number of sub-beams is 28 in thisembodiment. But, it is of course that the number of sub-beams is notlimited to this value.

The irradiation method with the beams is not limited to that in FIG. 23.An irradiation method as shown in FIG. 24A is also suitable. In thisexample, a plurality (two in the illustrated example) of lasers are eachdivided into a plurality (three in the illustrated example) ofsub-beams. These sub-beams are moved to scan without overlapping eachother. In this example, lateral movement in each scanning operation canbe made shorter in distance.

Further, for example, an irradiation method as shown in FIG. 24B can besuitably used also. In this example, each DPSS laser 41 emits one energybeam. The energy beams emitted from the respective DPSS lasers 41 aremoved without overlapping each other. This irradiation method iseffective particularly for peripheral circuits, which requirecrystallization by a higher energy. This irradiation method is, ofcourse, usable for the pixel region.

As a result of observing crystal grains in each beam line formed by themethod of FIG. 23, it was confirmed that polysilicon with its crystalgrain size of 300 nm was formed.

Fabrication of TFT

TFTs were fabricated using, as the active semiconductor film, asemiconductor film crystallized with the DPSS laser device of thisembodiment. The fabrication process of the TFTs was the same as that inthe first embodiment described with reference to FIGS. 8 to 11.

In this embodiment, the semiconductor film to be crystallized was formedinto a ribbon pattern in which 50 μm-wide ribbons were arranged atcertain intervals so as to match the pixel layout. The wavelength was532 nm. The output was 10 W. The stability of the energy beam was <0.1rms % of noise. The stability of the output was <±1%/h. The shape of theenergy beam was elliptic of 80 μm×20 μm. The scanning speed was 100cm/sec. Crystallization was performed under the above conditions.

When the mobility of a TFT fabricated through the same process as thatin the first embodiment described with reference to FIGS. 8 to 11 wasexamined, it was about 20 cm²/Vs. This value shows a performance thatcan be sufficiently put in practical use as a pixel transistor.

Modifications

Modifications of this third embodiment will be described below.

Here will be described, as shown in FIG. 25, efficient crystallizationmethods for selectively crystallizing the semiconductor film only at theregions where TFTs are to be formed.

FIGS. 26A and 26B are schematic views of irradiation systems of DPSSlaser devices used in modifications of this embodiment.

Referring to FIG. 26A, an irradiation system A is made up from a fixedmirror 61 for reflecting a sub-beam in a predetermined direction, and amovable mirror 62 for further reflecting the beam, which has beenreflected by the fixed mirror 61, in a predetermined direction toirradiate the target region. Such an irradiation system A is providedfor each of split sub-beams.

Referring to FIG. 26B, an irradiation system B is made up from a fixedmirror 63 for reflecting a sub-beam to a predetermined direction, acollimator lens 64, and a condenser lens 65 for condensing the beam,which has been reflected by the fixed mirror 63, through the collimatorlens 64 to irradiate the target region. Such an irradiation system B isprovided for each of split sub-beams.

In addition to moving the X-Y stage 43, the optical system shown in FIG.26A or 26B is equipped for each of the split sub-beams. Either opticalsystem is designed to scan only the region for forming each TFT. Thatis, the range of each sub-beam shifting is less than 100 μm at most.

While the X-Y stage 43 is rapidly moved, the optical system is turned onat each pixel position to crystallize the pixel portion. This bringsabout an improvement of the throughput.

In this embodiment, the amorphous silicon can be formed on the entiresurface of the substrate, or into a ribbon or island pattern. In anycase, it is of course that the portions to be irradiated with the laserbeam must be match the layout of the pixel region.

(Fourth Embodiment)

Next, the fourth embodiment of the present invention will be described.

Here, like the second embodiment, the construction of a DPSS laserdevice and then a crystallization method of a semiconductor film usingthe DPSS laser device will be described. The DPSS laser device of thisembodiment differs from that of the second embodiment on the point thatthe laser beam can be selectively applied to arbitrary portions asdescribed below.

In this embodiment, the a-Si film is not processed in advance into anisland pattern but it is left as in the non-patterned state. In thisstate, using an energy beam whose size is restricted to substantiallycorrespond to the width of each island (about 100 μm or less),irradiation is intermittently carried out while the X-Y stage is moved.Thus obtained are crystallized regions (molten regions) equivalent tothe respective islands in the first embodiment. In this manner, theproblems of damaging the glass substrate and peeling off the film can beavoided.

In case of application to LCDs, while the peripheral circuit region ishighly integrated and it requires TFTs with better crystallization andhigher mobility, the pixel region includes the areas for the respectiveTFTs at relatively large intervals and each TFT requires not so highmobility. But, the occupied area by the pixel region is far greater thanthat by the peripheral circuit region. So, in the pixel region, the X-Ystage is moved at a high speed (about several m/s) and crystallizationis discretely performed only at necessary portions, thereby considerablyimproving the throughput.

Construction of DPSS Laser Device

FIG. 27 is a schematic view showing the principal part of a DPSS laserdevice according to this fourth embodiment.

The DPSS laser device includes a solid state semiconductor LD excitinglaser 41 like the second embodiment, an optical system 71 having thefunction of a collimator and the function of a condenser, a chopper 72as intermittent emission means that is disposed in the optical paththrough which the energy beam reaches the a-Si film 70 on the glasssubstrate, has transmission (ON) areas 72 a and interruption (OFF) areas72 b for the energy beam, and can be rotated in the arrow direction toallow the energy beam to intermittently pass through, a mirror 73 forreflecting the energy beam having passed through an ON area 72 a, to theglass substrate, and an X-Y stage 43 (not shown), like the secondembodiment, that can be freely driven horizontally and vertically. Usingthis DPSS laser device, a CW laser light, e.g., Nd:YAG laser light (2ω,the wavelength: 532 nm) is shaped into a beam size of 20 μm×5 μm throughthe optical system 72.

In this case, it is efficient to scan in the arrow direction indicatedin FIG. 28. This is effective even to intermittent irradiation as thecase in FIG. 1, for example, not limited to intermittent irradiationwith the energy beam.

FIG. 28 is a schematic view showing an example of arrangement of TFTs inthe pixel region.

In this embodiment, each pixel size is 150 μm×50 μm and the TFT regionmay has a size of 10 μm×15 μm. After a SiO₂ buffer layer (thickness: 200nm) and then an a-Si film (thickness: 150 nm) are formed on a glasssubstrate, the chopper 72 is rotated. The energy beam is thereby madeON/OFF at a rate of 7.5 μs/17.5 μs, and applied with a scanning speed(speed of the X-Y stage) of 2 m/sec. In this manner, without processingthe a-Si film into an island pattern, without damaging the glasssubstrate and peeling off the film, only necessary portions of the a-Sifilm (e.g., crystallization regions 74 in FIG. 27) can be selectivelycrystallized.

Preferably in this case, the energy beam is intermittently applied tocertain portions of the a-Si film other than where TFTs are to beformed, to crystallize and form positioning markers 75 (FIG. 27) havinga predetermined shape, and these markers 75 (FIG. 27) are used asindexes upon crystallization of the a-Si film.

When crystallization of the a-Si film is performed by the above method,large-grain crystals can be obtained using the CW laser, and this doesnot bring about any increase in process steps and time. In a TFT formedwith such a large-grain crystal, the characteristics is improved andunevenness due to crystal is reduced. Thus, a high-performance liquidcrystal display device with values added can be provided with holdingdown the cost.

Modifications

Modifications of this fourth embodiment will be described below.

(Modification 1)

In this modification, a crystallizing method of the a-Si film for TFTsin the peripheral circuit region of a liquid crystal display device willbe described.

In the peripheral circuit region, the degree of integration is higherthan that in the pixel region, and requirement for crystallization issevere. As regions where TFTs are to be formed, crystallization regionseach having a size of 50 μm×200 μm are formed at intervals of 5 μm. Acircuit is formed in each of the crystallization regions. For thispurpose, a CW laser is shaped into a beam size of 50 μm×5 μm through theoptical system. After a SiO₂ buffer layer (thickness: 200 nm) and thenan a-Si film (thickness: 150 nm) are formed on a glass substrate, thechopper 72 is rotated. The energy beam is thereby made ON/OFF at a rateof 1 ms/0.025 ms, and applied with a scanning speed (speed of the X-Ystage) of 20 cm/sec. By reducing the scanning speed to about 20 cm/sec,fluent long crystal grains (flow pattern) can be obtained, therebyforming a TFT with a high mobility. In this manner, without processingthe a-Si film into an island pattern, without damaging the glasssubstrate and peeling off the film, high-quality crystals can be formedat necessary portions.

(Modification 2)

In this modification, the intermittent emission means for making theenergy beam ON/OFF is implemented by a combination of a small hole and amirror.

FIG. 29 is a schematic view showing the principal part of a DPSS laserdevice according to this modification 2.

In addition to the DPSS laser 41 and the optical system 71, the DPSSlaser device includes a rotatable mirror for reflecting the energy beamin a desired direction, in place of the chopper 72. The DPSS laserdevice further includes an interruption board 76 having a small hole 76a for allowing only the energy beam in a desired direction to pass. Inthis modification, the mirror 77 is rotated to swing the energy beam.The energy beam is made ON only when it passes the hole 76 a. As themeans for swing the energy beam, a rotatable polygon mirror can also beused.

(Modification 3)

FIG. 30 is a schematic view showing the principal part of a DPSS laserdevice according to this modification 3.

The DPSS laser device of this modification has almost the sameconstruction as that in the third embodiment, but differs on the pointthat the chopper 72 is processed and, attendant upon this, a number ofmirrors are disposed.

In this modification, predetermined ones of the ON areas 72 a of thechopper 72 are stopped with interruption plates 81 that can reflect theenergy beam. A number of mirrors 82 are provided for reflecting theenergy beam reflected on the respective interruption plates 81, inpredetermined directions. By this structure, the energy beam reflectedon an interruption plate 81 is turned to irradiate a neighboring line ofthe a-Si film 70 and further the next neighboring line. In case of apixel size of 50 μm×150 μm and a TFT region size of 15 μm×10 μm as shownin FIG. 28, about two-thirds of one scanning period is in the OFF state.By irradiating the neighboring two lines during this OFF period, threelines can be irradiated for every scanning period. Thus, the processtime can be shortened to about one-third.

During one scanning period with the X-Y stage 43, the non-irradiationtime is several times longer than the irradiation time, so the energybeam is rapidly moved to the neighboring line one after another duringthe non-irradiation time. The idle time can be reduced thereby and thethroughput can be improved.

As described above, according to this modification, by restricting theenergy beam of the CW laser to 100 μm or less and applying itintermittently, a large-grain crystal can be formed without damaging theglass substrate and peeling off the film. Besides, by irradiating theneighboring lines during the non-irradiation time of one line, theseveral lines can be crystallized during every scanning period, therebyimproving the throughput. This makes it possible to suppress unevennessof the TFT characteristics dependent upon crystal grain boundaries orsize, and good element characteristics can be obtained. As a result, ahigh-quality liquid crystal display device with its drive circuit beingincorporated therein can be provided.

(Modification 4)

FIG. 31 is a schematic view showing the principal part of a DPSS laserdevice according to this modification 4.

The DPSS laser device of this modification has almost the sameconstruction as that in modification 3, but differs on the point that apolygon mirror is used in place of the chopper 72.

In addition to the DPSS laser 41 and the optical system 71, the DPSSlaser device of this modification includes a polygon mirror 83 in placeof the chopper 72, and an interruption board 84 having a number of (inthe illustrated example, three) holes 84 a for allowing only the energybeams in predetermined directions to pass, in accordance with thedirections of the energy beam reflected on the polygon mirror 83.

In this modification, the polygon mirror 83 is rotated to swing theenergy beam, and thereby three lines on the a-Si film 70 are irradiatedat once during one scanning period. But, after the first line isirradiated, since the X-Y stage 43 has moved, the irradiation portion ofthe second line (the position of the hole 84 a) must be located at theposition advanced in accordance with the movement of the X-Y stage 43.The same applies also to the third line.

According to this modification, like modification 3, a large-graincrystal can be formed without damaging the glass substrate and peelingoff the film. Besides, by irradiating the neighboring lines during thenon-irradiation time of one line, the several lines can be crystallizedduring every scanning period, thereby improving the throughput.

On the fourth embodiment and its modifications 1 to 4, as shown in FIG.27 to FIG. 31, the energy beam is scanning the X-Y stage to be moved inthe bold arrow direction. When one line is scanned, the energy beam ismoved in the narrow arrow direction to scan next line.

(Fifth Embodiment)

Next, the fifth embodiment of the present invention will be described.

In this embodiment, for making TFTs, an a-Si film is crystallized with aCW laser, like the first to fourth embodiments. At this time, thisembodiment is aimed principally at preventing peeling-off of the a-Sifilm, which is caused by the temperature rising of a buffer layer due toirradiation with the energy beam. This embodiment discloses TFTs with aproper buffer layer.

It is known that the use of SiN or SiON as the material of the bufferlayer formed between the glass substrate and the a-Si film is effectivefor preventing contamination by impurities such as sodium from the glassconstituting the substrate. FIG. 32 shows a result obtained by examiningthe distribution of hydrogen in a buffer layer that is not modifiedafter it is formed.

When an a-Si film formed over a substrate with a buffer layer beinginterposed between them, which layer contains SiN or SiON, iscrystallized with an energy beam that generates energy continuously inrelation to time, in this example, a CW laser, because the buffer layerabsorbs the energy beam (or due to thermal conduction when the a-Si filmis melted), the temperature rises. When the density of hydrogen in thebuffer layer is high, effusion of hydrogen atoms occurs to generatepinholes, which causes peeling-off of the film. Also, when the densityof hydrogen in the a-Si film is high, effusion occurs to generatepinholes. When both the densities are high, as shown in FIG. 33, thea-Si film is peeled off due to pinholes. This phenomenon is remarkableparticularly when using a continuous energy beam, in comparison with thecrystallization using a conventional excimer laser.

So, in this embodiment, as shown in FIG. 34, an a-Si film 93 is formedover a glass substrate 91 with a buffer layer 92 being interposedbetween them. The buffer layer 92 is a multilayer film made up from athin SiN or SION film 92 a having a thickness of about 40 nm and a SiO₂film 92 b. Prior to crystallization of the a-Si film 93 with a CW laser,the density of hydrogen in either of the a-Si film 93 and the thin film92 a is regulated. More specifically, the density of hydrogen in thea-Si film 93 is regulated to 1×10²⁰/cm³ or less, and the density ofhydrogen in the thin film 92 a is regulated to 1×10²²/cm³ or less. Byproviding the SiO₂ film 92 b, interface state density between Si filmand buffer layer can be reduced. Besides, upon irradiation with theenergy beam of the CW laser, irradiation from the upper surface side ofthe substrate is preferable to irradiation from the lower surface sideof the substrate because SiN is not directly irradiated with the laserlight.

[Proper Density of Hydrogen in a-Si Film]

Here will be described an experimental result obtained by examining theproper density of hydrogen in the a-Si film.

As shown in FIG. 34, a thin SiN film 92 a having a thickness of about 50nm and then a SiO₂ film 92 b having a thickness of about 200 nm wereformed on a glass substrate 91 through a P-CVD process to form a bufferlayer 92. An a-Si film 93 having a thickness of about 150 nm was thenformed thereon. The thickness of each film is not limited to the abovevalue.

Subsequently, the a-Si film 93 was dehydrogenated by a thermal treatmentin a nitrogen atmosphere at 500° C. for two hours. After this,crystallization was performed with a semiconductor-exciting(LD-exciting) solid state laser (DPSS laser) Nd:YVO₄ under conditions ofan output of 6.5 W, a scanning speed of 20 cm/sec, and a wavelength of532 nm (the second harmonic of Nd:YVO₄). Scanning was performed bymoving the X-Y stage.

FIG. 35 is a graph showing an result of an SIMS analysis of the glasssubstrate/SiN/SiO₂/a-Si structure after the thermal treatment at 500° C.for two hours.

In this SIMS analysis, after the thermal treatment at 500° C. for twohours, it was confirmed that the density of hydrogen in the a-Si film 93became 1×10²⁰/cm³ or less.

FIG. 36 is a photomicrographic view showing the semiconductor film aftercrystallization.

It is found that a good crystal without pinholes and peeling-off couldbe obtained by regulating the density of hydrogen in the a-Si film 93 to1×10²⁰/cm³ or less.

[Proper Density of Hydrogen in SiN Film]

Next will be described an experimental result obtained by examining theproper density of hydrogen in the SiN film constituting the bufferlayer.

As shown in FIG. 34, a thin SiN film 92 a having a thickness of about 50nm and then a SiO₂ film 92 b having a thickness of about 200 nm wereformed on a glass substrate 91 through a P-CVD process to form a bufferlayer 92. An a-Si film 93 having a thickness of about 150 nm was thenformed thereon. The thickness of each film is not limited to the abovevalue.

Subsequently, the a-Si film 93 was dehydrogenated by a thermal treatmentin a nitrogen atmosphere at 450° C. for two hours. In a SIMS analysis,it was confirmed that the density of hydrogen in the thin SiN film 92 abecame 1×10²²/cm³ or less. At this time, the density of hydrogen in thea-Si film 93 was 1×10²⁰/cm³ or less.

The a-Si film 93 was then crystallized with a semiconductor-exciting(LD-exciting) solid state laser (DPSS laser) Nd:YVO₄ under conditions ofan output of 6.5 W, a scanning speed of 20 cm/sec, and a wavelength of532 nm (the second harmonic of Nd:YVO₄). Scanning was performed bymoving the X-Y stage. As a result, a good crystal was obtained like inFIG. 36.

As described above, according to this embodiment, it becomes possible tounformize the transistor characteristics of TFTs at a high level usingcrystallization with an energy beam that outputs energy continuously inrelation to time, and to stably form the TFTs without generation ofpinholes and peeling-off, thereby realizing the very highly reliableTFTs.

In the above-described embodiments, the a-Si film is used as thesemiconductor film. However, the present invention is applicable also toany case wherein the initial film is a p-Si film formed through an LPCVDprocess, a p-Si film by solid phase growth, a p-Si film by metal-inducedsolid phase growth, or the like.

The present invention can implement TFTs in which the transistorcharacteristics of the TFTs are made uniform at high level, and themobility is high particularly in the peripheral circuit region to enablehigh-speed driving in applications to a peripheral circuit-integratedTFT-LCD, system-on panel, system-on glass, and the like.

Further, according to the present invention, it becomes possible thatthe insufficiency of the output of an energy beam, which outputs energycontinuously in relation to time, is compensated so that the throughputin crystallization of semiconductor films is improved, thereby realizinghighly efficient TFTs whose mobility is high particularly in aperipheral circuit region to enable high-speed driving.

What is claimed is:
 1. A semiconductor device comprising: a substrate; apixel region formed on said substrate and including thin filmtransistors; and a peripheral circuit region about said pixel region,said peripheral circuit region being formed on said substrate andincluding thin film transistors, wherein each of active semiconductorfilms of said thin film transistors in said peripheral circuit region isformed into a crystalline state having a streamlined flow pattern withlarge crystal grains.
 2. The device according to claim 1, wherein thegrain size of said flow pattern is longer than a channel length.
 3. Thedevice according to claim 2, wherein said active semiconductor filmshave been formed by applying an energy beam to semiconductor filmspatterned on said substrate into lines or islands.
 4. The deviceaccording to claim 3, wherein markers for positional adjustment ofirradiation with said energy beam are provided on said substrate tocorrespond respectively to said semiconductor films patterned.
 5. Thedevice according to claim 1, wherein said active semiconductor films insaid peripheral circuit region are different in thickness from activesemiconductor films in said pixel region.
 6. The device according toclaim 1, wherein said substrate is made of one selected from the groupof non-alkali glass, quartz glass, ceramic, plastic, and monocrystallinesilicon with amorphous insulating film.
 7. The device according to claim1, wherein a semiconductor film is formed over said substrate with abuffer layer being interposed between them, said layer including a thinfilm containing Si and N, or Si, O, and N, and the density of hydrogenin said semiconductor film is not more than 1×10²⁰/cm³.
 8. The deviceaccording to claim 7, wherein the density of hydrogen in said thin filmis not more than 1×10²²/cm³.
 9. The device according to claim 7, whereinsaid buffer layer has a structure of SiO₂/SiN or SiO₂/SiON.